The more space the better.
It is popular on this particular forum to quote 4 sq ft per hen indoors, plus 8-10 sq ft per hen in the run (or free range of course), as some sort of guideline. (Other forums authorities authors etc have their own in-house conventions, some for less space, a few for more).
This stocking density will keep the chances of cannibalism fairly low as long as the chickens are getting out a good deal most days. (Not as reliably successful in very cold winter areas, though some are lucky enough for it to go ok)
BUT, there is a lot of difference in HOW chickens behave at that stocking density vs when given more space. I find them to act much more relaxed, less-stressed, happier, more "natural" and individual, at much more generous space allotments e.g. 15 sq ft per chicken indoors plus ample run. And personally I value that and would not keep them in the common more-crowded densities.
it just depends what you're after.
(And also what your climate is like!)
Pat